Ductile high temperature tungstenrhenium alloy and process for making same

ABSTRACT

A NOVEL DUCTILE HIGH-TEMPERATURE TUNGSTEN-BASE ALLOY CONTAINING 3 TO 20% RHENIUM AND .05 TO 3.0% YTTRIUM OR MIXTURES OF YTTRIUM AND CERIUM IS MADE BY COMPACTING POWDERS OF THE INGREDIENT ELEMENTS, HEATING THE COMPACT AT AN INTERMEDIATE TEMPERATURE WELL BELOW THE COMPACT&#39;&#39;S SINTERING TEMPERATURE IN A HYDROGEN ATMOSPHERE TO EFFECT REMOVAL OF INTERSTITUALS, AND THEN SINTERING THE COMPACT IN A DYNAMIC VACUUM.

United States Patent 3,573,903 DUCTILE HIGH TEMPERATURE TUNGSTEN-RHENIUM ALLOY AND PROCESS FOR MAK- ING SAME Eugene J. Delgrosso,Wallingford, Conn., assignor to United Aircraft Corporation, EastHartford, Conn. No Drawing. Filed July 21, 1964, Ser. No. 385,101 Int.Cl. C22c 27/00 US. Cl. 75176 11 Claims ABSTRACT OF THE DISCLOSURE Anovel ductile high-temperature tungsten-base alloy containing 3 to 20%rhenium and .05 to 3.0% yttrium or mixtures of yttrium and cerium ismade by compacting powders of the ingredient elements, heating thecompact at an intermediate temperature Well below the compacts sinteringtemperature in a hydrogen atmosphere to effect removal of interstitials,and then sintering the compact in a dynamic vacuum.

This invention relates to novel tungsten-base alloys having tungsten astheir principal component and to a method for making such alloys.

More particularly, this invention relates to novel tungsten-rheniumalloys containing small amounts of yttrium or yttrium and cerium. Suchalloys are ductile at room temperature and are capable of being formedinto tube, sheet and other configurations by conventional fabricationmethods.

These alloys are created by compacting powders of the ingredientelements, heating the powdered compact at an intermediate temperaturewell below the compacts sintering temperature in a hydrogen atmosphereto etfect removal of interstitials, and then sintering the compact in adynamic vacuum.

Tungsten-rhenium alloys have been well known for many years. Theprincipal use of such alloys has been in thermocouples for measuringunusually high tempera tures. A typical tungsten-rhenium alloy for usein such thermocouples is tungsten-30% rhenium by weight. Normally, suchthermocouples comprise one leg consisting of W-30% Re by weight (W-30Re)and a second leg consisting of pure tungsten or a low rhenium contentalloy such as W-3 to 6% Re by weight.

Tungsten-rhenium thermocouples are used to control temperatures inhigh-temperature vacuum furnaces, rupture rigs and reactors. They aregenerally considered to be the best commercially availablehigh-temperature thermocouples.

Rhenium has also been used as a solid solution softener of tungsten.Because of its high melting point and ability to retain high strength athigh temperatures, tungsten potentially is one of the best alloy basesfor high-temperature structural uses in nuclear fuel containmentvessels, thermionic emitters, reactors, rockets and electrical andelectronic devices.

Great problems have been encountered, however, in atempting to usetungsten in forming structural parts because of its brittleness at roomtemperature and temperatures at which it can be conveniently workedusing conventional fabrication methods, Although pure tungsten exhibitsductility in the lower temperature ranges, tungsten has great affinityfor oxygen and nitrogen, which leads to its rapid embrittlement wheneverit is exposed to air.

In amounts up to about 50% by weight rhenium forms a solid solution withtungsten. It has been known for some years that rhenium in amounts of25% or more by weight, when added to tungsten in solid solution,counteracts the embrittling effect of oxygen and nitrogen inter-3,573,903 Patented Apr. 6, 1971 "ice stitials in the tungsten andpromotes ductility in the resulting alloy. It is believed that rheniumcauses the oxygen and nitrogen in the grain boundaries to formdiscontinuous globules in place of the brittle continuous grain boundaryenvelope-type films normally formed by oxygen and nitrogen asinterstitials in pure tungsten. These rhenium induced globules permitslippage between grain boundaries and result in an alloy exhibitingductility at temperatures down to and including room temperature.

In recent years this ductility improving characteristic of rhenium hasresulted in development of semicommercial W-Re alloys that retain.ductility at room temperature. Such alloys, however, must contain atleast 25% rhenium by weight to effectively counteract the embrittlingoxygen and nitrogen interstitials. Semicommercial W-25Re alloys has beenused for making both wire and thermocouples. Potentially, it is also acandidate for the manufacture of high-temperature structural parts.

Unfortunately, however, rhenium is a very rare metal. Its cost currentlyruns as high as $1500 per pound. Since the semicommercial alloy requiresat least 25 rhenium by weight in solid solution with tungsten to achieveroom temperature ductility, the cost of suitable tungsten-rhenium alloysfor structural materials is almost prohibitive.

It is accordingly a primary object of this invention to providetungsten-rhenium alloys and a method for making them in which therhenium content can be reduced to amounts of as little as 3% rhenium byweight without sacrificing room temperature ductility.

Another object of this invention is to provide a novel and improvedtungsten-rhenium alloy containing small amounts of rhenium and evensmaller amounts of yttrium or a mixture of yttrium and cerium thatexhibits room temperature ductility and permits fabrication ofstructural parts by conventional methods at moderate fabricatingtemperatures, This invention thus achieves tungsten alloys containing aslittle as 3% by weight rhenium that can be fabricated into tube andsheet type structural parts by conventional fabrication methods.

It is another object of this invention to provide a new and improvedprocess for producing a ductile tungstenrhenium alloy containing smallamounts of rhenium that simultaneously achieves purification of thetungsten base and formation of a ductile tungsten-rhenium alloy.

Yet another object of this invention is to provide a new and improvedtungsten-rhenium alloy and process for making it that yields an alloyhaving distinctly improved resistance to corrosion at high temperatures.The alloys of this invention thus have exceptionally improved resistanceto corrosion by the alkali metals, particularly liquid lithium, whenused as structural tubing in reactors.

A further object of this invention is to provide a new and improvedtungsten-rhenium alloy containing small amounts of rhenium and evensmaller amounts of yttrium in which yttrium promotes thermionicproperties in the alloy and achieves an alloy having enhanced value as athermionic emitter or thermionic converter over conventionaltungsten-rhenium alloys.

The high amounts of rhenium required in prior art tungsten-rheniumalloys to achieve ductility degrade the overall strength of such alloys.It is thus an important object of the present invention to provide atungstenrhenium alloy that attains ductility with the use of much lessrhenium than is required in prior art alloys. The duetile tungsten-basealloys of this invention accordingly have appreciably greater strengthand are must less expensive than similar prior art alloys.

Additional objects and advantages will be set forth in part in thedescription that follows, and in part will be obvious from thedescription, or may be learned by practice of the invention, the objectsand advantages being realized and attained by means of the compositions,

methods and processes particularly pointed out in the appended claims.

To achieve the foregoing objects in accordance with its purpose, thisinvention includes a high-temperature ductile alloy consistingessentially of tungsten as the principal component, from 3 to 20% byweight of rhenium, and from .05 to 3% by Weight of a metal additiveselected from the group consisting of yttrium and mixtures of yttriumand cerium.

The invention further comprehends a process of pro ducing ahigh-temperature ductile alloy consisting essentially of tungsten as theprincipal component, from 3 to 20% by weight of rhenium, and from .05 to3% by weight of a metal additive selected from the group consisting ofyttrium and mixtures of yttrium and cerium. The process comprises thesteps of: compacting a mixture of finely divided powders consistingessentially of from 3 to 20% by weight of rhenium, .05 to 3% by weightof a metal additive selected from the group consisting of yttrium andmixtures of yttrium and cerium and the balance essentially tungsten,heating the resultant compact in a hydrogen atmosphere at a temperatureof from 1400 to 2700 F. for a time period sufficient to achieveoutgassing of a substantial portion of oxygen and nitrogen interstitialsand to impart a density of from 55 to 75% in the compact, sintering thecompact in a dynamic vacuum at a temperature of from 3600 to 4200 F. fora time period sufficient to achieve from 85 to 9 8% density in thecompact, thereby creating a ductile tungsten-base alloy having highstrength at high temperatures and fabricability at low and moderatetemperatures.

For convenience, the first heating step is termed a presintering stepand the second heating step is termed a densifying or sintering step.

In a preferred form, the invention also includes within its scope aductile high-temperature alloy consisting essentially of tungsten as theprincipal component, from to 15% by weight of rhenium, and from .05 to3% by weight of a metal additive selected from the group consisting ofyttrium and mixtures of yttrium and cerium.

When a mixture of yttrium and cerium is used, the ratio by weight ofyttrium to cerium should be from 1:2 to 2: 1. A preferred ratio is 1:1.When the mixture is cerium rich, the compact should be heated up moreslowly during the presintering step than when the mixture is yttriumrich.

The tungsten, rhenium, yttrium and cerium powders used to produce thealloys of this invention should be of the highest purity practicallyattainable and usually have a size range of less than 300 mesh (48microns) although coarser particles, ranging in size from about 100 to300 mesh (147 to 48 microns) may also be used. Especially good resultsare obtained when the size range of the powders is less than 400 mesh(38 microns), or between about 0 to 38 microns and preferably betweenabout 0 to 10 microns. In general, it can be said that the liner theparticles, thebetter the alloy compact produced. The mesh sizes reportedare Tyler standard.

There has been a long felt need for achieving a ductile tungsten alloythat would be capable of being formed into structural parts such as tubeand sheet at moderate Working temperatures and that would not requirelarge amounts of rare and expensive rhenium to impart desired ductility.This invention achieved the unexpected new and useful result of makingpossible a dramatic reduction in rhenium content of tungstenrheniumalloys and yet still attains the desired room-temperature ductility bythe addition of small amounts of yttrium or yttrium and cerium to thecharge used to produce the alloy and by controlled process steps used informing the alloy.

In the process of producing the alloy the yttrium or yttrium and ceriumfunction to eliminate from the tungsten a major proportion of its oxygenand nitrogen interstitials. The removal of these interstitials in turnmakes it possible to achieve desired ductility in the alloy with 4 muchsmaller amounts of rhenium than would otherwise be required. With thetungsten base freed of most of its interstitials, the small proportionof rhenium used in the alloys of this invention is sufficient tocounteract the embrittling effect of any remaining interstitials.

The yttrium content of the alloy provides an additional unexpected newand useful result in reducing the susceptibility of the alloy tocorrosion attack by liquid alkali metals, particularly lithium, whenused as structural members for reactors or nuclear containment vessels.Yttrium getters interstitial oxygen and forms oxides more stable thanlithium oxide; this prevents leaching of oxygen by lithium (or otheralkali metals) from less stable rhenium oxides.

The tungsten-rhenium alloy resulting from the process of this inventionpossesses high ductility and can be readily formed into structuralcomponents, such as tubing or sheet, at room or moderate workingtemperatures. Since substantial amounts (usually about one-half or moreby Weight) of the yttrium, or yttrium and cerium, originally present aspart of the alloy charge may be evolved during the process as vaporphase compounds with oxygen and nitrogen, it sometimes occurs that onlyminor amounts of yttrium and cerium will be found inthe final product.Cerium, partly because of its lower melting point, is particularlysusceptible to loss in vapor phase during the presintering and sinteringsteps.

Accordingly, the charge for producing the alloys of this inventionshould contain from 1 to 5% by weight of yttrium or a mixture of yttriumand cerium to yield an alloy product containing from .05 to 3% by weightof yttrium or a mixture of yttrium and cerium.

In carrying out the process of this invention, the ingredient elementsin the form of metal powders are mixed and compacted in an inertatmosphere. Typically, this can be accomplished in a dry box that isevacuated and back filled with argon, preferably to a little belowatmospheric pressure. The powders are then compacted in the inertatmosphere, preferably at a pressure of from 25 to 60 TSI.

The pressed compacts are placed in a protective carrying container andtransferred to a high-temperature resistance furnace. The presinteringstep is carried out in this furnace in which the compacted powders ofthe alloys are exposed to a high purity hydrogen atmosphere at from 1400to 2700 F. for a time period sufiicient to evolve a major proportion ofthe various gaseous interstitials in the compact either directly asocculded or adsorbed gases or as vapor phase compounds with yttrium oryttrium and cerium. Preferably, the furnace is operated at about 2200 F.and the desired amount of outgassing can be accomplished in about 2hours. It is important, however, that the furnace be raised slowly totemperature, since this promotes the release of occluded or adsorbedgases and evolution of vapor phase oxides and nitrides of yttrium andcerium at a steady rate, avoids cracking and fracturing of the compact,and maintains porosity permitting continuous evolution of gases.

In this presintering step the compact achieves a green density of from55 to 75%, preferably 60 to 70%. The presintering thus gives body to thecompact and holds it in its compacted shape. Most of the outgassing ofinterstitials occurs during this presintering step and improves theresults obtained in the densifying step which follows.

Both yttrium and cerium perfonm gettering functions in the presinteringstep, since they readily tend to form volatile compounds with oxygen andnitrogen and exhibit a preferential afiinity for the oxygen and nitrogenpresent as interstitials in the tungsten. This results in simultaneouspurification and improvement of ductility in the resultant alloy.

Cerium has a lower melting point than yttrium and cerium oxide isvolatile at a lower temperature than yttrium oxide. Accordingly, whencerium and yttrium are used together as components of the alloy, thecompact during the presintering step may be heated to the melting pointof cerium or about 1460 F. and held at that temperature for a timeperiod sufficient to remove quantities of oxygen and nitrogen as vaporphase compounds with cerium, and the temperature can then be slowlyincreased toward the melting point of yttrium (about 2700 F.),preferably to about 2200 F., to promote evolution of oxygen and nitrogenas vapor phase compounds with yttrium.

-In accordance with the invention, residual yttrium remaining in thealloy after sintering improves the overall corrosion resistance of thealloy by its gettering of oxygen to form stable yttrium oxides.

Upon completion of the presintering step the compact is transferred to avacuum atmosphere furnace capable of maintaining a dynamic vacuum. Thecompact is then slowly heated to a temperature of from 3600 to 4200 F.with the pumps on and held at that temperature for a time periodsufiicient to achieve 85 to 98% density in the compact. Generally, thiswill be from about 1 to 4 hours.

'During sintering in the vacuum atmosphere the pressure may rise toabout 1000 microns due to continued outgassing of occluded and adsorbedgases and evolution of vapor phase compounds of yttrium and cerium withoxygen and nitrogen. As the sintering step nears completion, the vacuumwill be observed to fall back to a pressure of from to 1 microns. Whenthe pressure has dropped off to about 10 microns or less, this is anindication that densifying or sintering has neared or reachedcompletion. At this point the heat may be shut off and the furnaceallowed to cool to ambient temperature, when the compact is removed.

The resultant alloy compacts of this process exhibit good warm or coldworkability or fabricability and can be treated in much the same manneras the semico-mmercial W-30Re alloys, even though they may contain aslittle as 3% by weight of rhenium. For example, the alloys of thisinvention can be cold or warm swaged or cold or warm forged and thenrolled hot or cold.

In accordance with the invention, the additions of yttrium or yttriumand cerium. are used to remove almost all of the interstitials fromtungsten. Most of the compositional constituents that contribute to theembrittlement of tungsten are eliminated and the embrittling effects ofthe remaining interstitials are overcome with a much smaller amount ofrhenium than has been required with prior tungsten-rhenium alloys.

In this way ductile tungsten alloys can be created using small amountsof rhenium to achieve a stronger and much less expensivehigh-temperature tungsten-base alloy than has heretofore been possible.Moreover, such ductile tungsten-base alloys can be fabricated at lowtemperatures without the necessity of resorting to expensive, elaborateand time-consuming preparation processes necessary for high-temperaturefabrication of normally brittle tungsten-base alloys, such as sheathing,canning, or sheltering in an inert atmosphere.

For a clearer understanding of the invention, specific examples are setforth below. These examples are merely illustrative and are not to beunderstood as in any way limiting the scope and underlying principles ofthe invention.

EXAMPLE 1 An intimate mixture of high purity metallic powders of thefollowing composition was carefully prepared in a dry box under an argonatmosphere by milling or V blendmg:

94 grams of tungsten powder (-300 mesh or finer), 5 grams of rheniumpowder (-300 mesh or finer), 1 gram of yttrium powder (-300 mesh orfiner).

The blended powders were then pressed in a 1 inch diameter breakawaytype die at 50 T81. The compact was next inverted and repressed in thesame manner. The blending and compacting were all performed in an argonatmosphere to avoid atmospheric contamination.

The pressed compact was placed in a high-temperature resistance furnacefor presintering. The furnace was evacuated and backfilled with highpurity hydrogen to a slight positive pressure. It was then slowly heatedto promote outgassing of occluded and adsorbed gases from the surfacesof the particles forming the compact and outgassing of vapor phasecompounds of yttrium with interstitials in the constituent elements ofthe compact.

The temperature in this presintering furnace was slowly raised to 2200F. and held there for 2 hours. At the end of 2 hours the furnace wasallowed to cool to room temperature and the compact was removed. By thispresintering step the compact achieved a density of 7 about 65%.

The compact was next placed in a high-temperature vacuum resistancefurnace for the densifying step. In this step the temperature was againslowly raised with the pumps on to about 4000 F. and was held at thattemperature for about 3 hours. During this step the pressure in thefurnace rose to about 1000 microns and then slowly fell back to about 10microns.

When the pressure reached 10 microns at the end of about 3 hours, thefurnace was turned off and allowed to cool to room temperature and thecompact withdrawn. The rise in pressure within the dynamic vacuumfurnace indicated that there was a continuous evolution of gases fromthe compact including vapor phase compounds of yttrium with oxygen andnitrogen.

Upon removal from the furnace the compact was found to have achieved adensity of about It was then warm swaged at about 1300 F., withreduction of about 15% per pass. Intermediate anneals at 3000 F. weregiven as required to avoid cracking, and in this manner the compact wasreduced from 1 inch to inch diameter. This inch rod was then cold swagedat room temperature with reductions of about 10% per pass and anintermediate anneal was given after each pass. The resultant wire wasfree of slivering and exhibited room temperature ductility.

By contrast, wire produced from W-5Re alloy having no yttrium in thecharge from which the alloy was produced silvered badly evidencingembittlement. Also, grain boundary hardness of the alloy of this examplewas only one half that of a W-5Re alloy without yttrium.

If warm working is not desired, the alloy of this example can be coldworked directly in the same manner as described for the warm working.This fabrication at room temperature can be accomplished easily andwithout the formation of injurious cracks, slivers or other evidence ofembrittlement. It is apparent from the attributes exhibited that thealloy of this example retains significant ductility at room temperature.

The resultant alloy had the following composition in percent by weight:

Tungsten 94.5 Rhenium 5 Yttrium 0.5

Significantly, about one-half the original charged weight of yttrium waslost in the course of forming the alloy. This yttrium loss is accountedfor by the formation and pumping off of vapor phase compounds of yttriumwith interstitial oxygen and nitrogen.

EXAMPLE 2 A compact was prepared as in Example 1 using an intimatemixture of high purity metallic powders having the followingcomposition:

89 grams of tungsten powder (-300 mesh or finer), 10 grams of rheniumpowder (-300 mash or finer). 1 gram of yttrium powder (-300 mesh orfiner).

This yielded an alloy of the following composition in percent by weight:

Tungsten 89.5 Rhenium 1 Yttrium 0.5

Results similar to Example 1 were obtained.

' EXAMPLE 3 An alloy having the following composition in percent byweight was prepared as in Example 1:

Tungsten 84.5 Rhenium 15 Yttrium 0.5

Results similar to Example 1 were obtained.

EXAMPLE 4 An alloy having the following composition in percent by weightas in Example 1 was prepared:

Tungsten 79.5

Rhenium 20 Yttrium 0.5

Results similar to those of Example 1 were obtained.

EXAMPLE 5 A mixture of high purity metallic powders of the followingcomposition was prepared:

93 grams of tungsten powder (300 mesh or finer), 5 grams of rheniumpowder (-300 mesh or finer), 1 gram of yttrium powder (300 mesh orfiner),

1 gram of cerium powder (300 mesh or finer).

An alloy was prepared from this composition as set forth in Example 1.This alloy had the following composition percent by weight:

Tungsten 94.2 Rhenium 5 Yttrium 0.5

Cerium 0.5

Results similar to those of Example 1 were obtained.

EMMPLES 68 The following additional alloys were prepared as inExample 1. Compositions are expressed in terms of percent by weight:

Results similar to those of Example 1 were obtained with each of thesealloys exhibiting room temperature ductility and fabricability.

. EXAMPLE 9 A mixture of high purity metallic powders of the followingcomposition was prepared:

93 grams of tungsten powder (-300 mesh or finer),

5 grams of rhenium powder (-300 mesh or finer), 2 grams of yttriumpowder (--300 mesh or finer).

8 An alloy was prepared from this composition as set forth in Example 1.This alloy had the following composition in percent by weight:

Tungsten 94 Rhenium 5 Yttrium 1 Results similar to those of Example 1were obtained.

EXAMPLES 10-12 The following additional alloys were prepared as inExample 1. Compositions are expressed in terms of percent by weight:

Results similar to those of Example 1 were obtained with each of thesealloys exhibiting room temperature ductility and fabricability.

The invention in its broader aspects is not limited to the specificdetails shown and described but departures may be made from such detailswithin the scope of the accompanying claims without departing from theprinciples of the invention and without sacrificing its chiefadvantages.

What is claimed is:

1. A ductile high-temperature alloy consisting essentially of tungstenas the principal component, from 3 to 20% by weight of rhenium, and from.05 to 3% by weight of a metal additive selected from the groupconsisting of yttrium and mixtures of yttrium and cerium.

2. A ductile high-temperature alloy consisting essentially of tungstenas the principal component, from 5 to 15% by weight of rhenium, and from.05 to 3% by weight of a metal additive selected from the groupconsisting of yttrium and mixtures of yttrium and cerium.

3. A ductile high-temperature alloy consisting essentially of tungstenas the principal component, from 3 to 20% by weight of rhenium, and from.05 to 3% by weight of yttrium.

4. A ductile high-temperature alloy consisting essentially of tungstenas the principal component, from 3 to 20% by weight of rhenium, and from.05 to 3% by weight of a mixture of yttrium and cerium, the ratio byweight of yttrium to cerium being from 1:2 to 2:1.

5. A ductile high-temperature alloy consisting essentially of tungstenas the principal component, from 3 to 20% by weight of rhenium, and from.05 to 3% by weight of a mixture of yttrium and cerium.

6. A ductile high-temperature alloy consisting essentially of tungstenas the principal component, from 5 to 15% by weight of rhenium, and from.05 to 3% by weight of yttrium.

7. A ductile high-temperature alloy consisting essentially of tungstenas the principal component, from 5 to 15% by weight of rhenium, and from.05 to 3% by weight of a mixture of yttrium and cerium.

8. A ductile high-temperature alloy consisting essentially of tungstenas the principal component, from 5 to 15% by weight of rhenium, and from.05 to 3% by weight of a mixture of yttrium and cerium, the ratio byweight of yttrium to cerium being from 1:2 to 2:1.

9. The process of producing a ductile high-temperature tungsten-basealloy comprising the steps of: compacting a mixture of finely dividedpowders consisting essentially of from 3 to 20% by weight of rheniurn,.05 to 3% by Weight of a metal additive selected from the groupconsisting of yttrium and mixtures of yttrium and cerium, and thebalance tungsten, heating the resultant compact in a hydrogen atmosphereat a temperature of from 1400 to 27 00 F. for a time period sufficientto achieve outgassing of a substantial portion of occluded and adsorbedgases and vapor phase compounds of oxygen and nitrogen with yttrium andcerium and to impart a density of from 55 to 75% in the compact,sintering the compact in a dynamic vacuum at a temperture of from 3600to 4200 F. for a time period suflicient to achieve from 85 to 98%density in the compact, whereby a ductile tungsten-base alloy havinghigh strength at high temperatures and fabricability at low to moderatetemperatures is created.

10 10. The process of claim 9, in which the compact is heated in ahydrogen atmosphere to a temperature of about 2200 F. for 2 hours.

11. The invention as defined in claim 9, in which the compact issintered in a dynamic vacuum at a temperature of about 4000 F. for from1 to 4 hours.

References Cited UNITED STATES PATENTS 10 3,138,453 6/1964 Poster et a1.75176X v FOREIGN PATENTS 816,135 7/1959 Great Britain 75-176 15 BENJAMINR. PADGE'IT, Primary Examiner R. L. TATE, Assistant Examiner US. Cl.X.R. 75-221, 225

